I would like some help to understand my CH/HW configuration.

My system is a totally standard open vented fully pumped Y-plan system
with an indirect mains pressure HW cylinder and 5 radiators. However,
there is one strange anomoly.

As you might expect the mid position valve is on the flow pipe from the
boiler, and its two outputs branch off to the Hot Water and the
Radiator loop.

However, there is a 2-way valve positioned between the HW port of the
mid-position valve and the connection to the Flow port of the HW tank
coil. This valve opens when the Call for Heat signal is given to the
boiler, i.e. HW, CH or Both are demanded by the controller.

At first glance this extra 2-way valve seems completely unnecessary.
Why has it been put here? What harm can it do? I initially thought it
could not do any harm if I leave it as it is. However, as the 2-way
valve is connected to the Call for Heat signal to the boiler and not
the live signal from the boiler to the pump, it could present a
problen.

For those that understand 3-way valves, if the valve gets left in the
HW only position, and the boiler overheat stat fires up the pump,
(which it does do for a few seconds every time the boiler has been on
for a while then subsequently turned off), the pump will be asked to
circulate the water in the HW coil, but won't be able to because the
extra 2-way valve is shut, therefore straining the pump.

Is this worth worrying about?

Thanks for your help

Luke

Thank you for that Christian.

I understand what you say and see why it is used as a safety device.

However I can't immediately work out how to wire the 2-way valve
correctly as you advise, i.e. to the HW On signal only. As you can
imagine the HW on signal from the Tank Stat is wired to Call for Heat
on the boiler. However, the orange output wire from the 3-way valve is
also wired to Call for Heat on the Boiler (live when CH is ON).
Therefore, I can't see how I could configure the 2-way valve correctly
so the it opens ONLY when the HW is on without using some sort of logic
gates. To be honest, it's probably not doing any harm being open when
the CH is also on, as the 3-way valve will be in CH only mode.

Okay, the bypass valve. As I understand it, this caters for when the
mid-position valve finds itself on CH only (i.e. grey wire - HW Off -
is live) and the overheat stat activates the pump. It must be fitted
between the pump and the mid-position valve and connect to the Return
pipe to the boiler. Could you just confirm that this is correct.

Thanks again
Luke

Sorry I meant:

Okay, the bypass valve. As I understand it, this caters for when the
mid-position valve finds itself in a relaxed state (HW only) and the
overheat stat activates the pump. It must be fitted between the pump
and the mid-position valve and connect to the Return pipe to the
boiler. Could you just confirm that this is correct.

In an earlier contribution to this discussion,
Wingedcat <(E-Mail Removed)> wrote:

> Thank you for that Christian.
>
> I understand what you say and see why it is used as a safety device.
>
> However I can't immediately work out how to wire the 2-way valve
> correctly as you advise, i.e. to the HW On signal only. As you can
> imagine the HW on signal from the Tank Stat is wired to Call for Heat
> on the boiler. However, the orange output wire from the 3-way valve is
> also wired to Call for Heat on the Boiler (live when CH is ON).
> Therefore, I can't see how I could configure the 2-way valve correctly
> so the it opens ONLY when the HW is on without using some sort of
> logic gates. To be honest, it's probably not doing any harm being
> open when the CH is also on, as the 3-way valve will be in CH only
> mode.
>
It sounds as if it has been wired to the cylinder stat's 'call for heat'
contact in the (mistaken) belief that it will only open when the HW temp is
below the stat set point. But whoever did it was ignorant of how Y-Plan
works, and didn't realise that it would also open in CH mode!

You need to isolate this valve from the rest of the Y-Plan electrics with
one of the following:
* Use a double pole cyl stat (if such a thing exists) - with one set of
contacts used in the normal Y-Plan fashion, and the other set driving this
valve
* Use a second cylinder stat to drive the valve - maybe set slightly higher
than the main stat - but well below the safe working temp of the cylinder
* Use the existing stat to drive a 2-pole relay, and use the 2 poles as per
double-pole stat, above

> Okay, the bypass valve. As I understand it, this caters for when the
> mid-position valve finds itself on CH only [corrected to HW only in
subsequent post] (i.e. grey wire - HW Off -
> is live) and the overheat stat activates the pump. It must be fitted
> between the pump and the mid-position valve and connect to the Return
> pipe to the boiler. Could you just confirm that this is correct.
>
Yes, that's where to connect it.

As an alternative, since you are fitting a by-pass anyway, why not get an
additional 2 port valve and put it in the CH circuit and convert the whole
thing to S-Plan. Replace the 3-port valve with a simple tee-piece. It then
gets a whole lot easier to understand - and avoids the need for any of the 3
solutions listed above!
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

Thank you both Christian and Set Square for your extremely useful help.

Firstly it is not possible to put in a second stat tank because it has
to be a special tank stat specially accommodated into the unvented
cylinder. I don't fancy this anyway in case the safety stat fails in
the open position then the safety two way valve will never open.

I think the way to go is to convert to S-plan and then I will have
independent control over HW and CH making connecting up the safety two
way valve much easier. Of course, it will involve a bit of modification
to the pipework but nothing major. Bearing in mind I will have to rejig
it a bit to take into account the bypass valve, I think this is the way
to go.

I also understand that I need a further 2 two-ways valves leaving the
safety in place.

Interesting point about the immersion heater requiring two thermostats.
There is indeed an immersion heater fitted to the tank but whether it
contains two thermostats internally I don't know. The cord to it simply
terminates in a 13 amp plug which is kicking about on the floor of the
airing cupboard not plugged in (there is no socket for it in the
vicinity).

Thanks again to you both.

Luke

"Christian McArdle" <(E-Mail Removed)> wrote:

>> For those that understand 3-way valves, if the valve gets left in the
>> HW only position, and the boiler overheat stat fires up the pump,
>> (which it does do for a few seconds every time the boiler has been on
>> for a while then subsequently turned off), the pump will be asked to
>> circulate the water in the HW coil, but won't be able to because the
>> extra 2-way valve is shut, therefore straining the pump.
>
>It is an essential safety feature designed to prevent a major explosion in
>the event of a boiler fault. It would unlawful to remove it. It appears to
>be miswired. It should be wired to a stat on the cylinder and only open when
>the cylinder calls for heat.
>
>It is not permitted for safety reasons to use a mains pressure cylinder as a
>boiler bypass as it can result in overheating of the cylinder. The separate
>valve enforces this restriction. If your system requires a bypass, then fit
>a separate automatic bypass valve to give the water somewhere to go.
>

So why does the standard Y plan diagram thats been around for years
never show this extra valve?


--

>So why does the standard Y plan diagram thats been around for years
>never show this extra valve?

Because this is an unvented mains pressure hot water cylinder which
requires the extra valve. Most hot water cylinders are gravity fed from
a loft tank so this extra safety precaution is not required.

"Wingedcat" <(E-Mail Removed)> wrote:

>>So why does the standard Y plan diagram thats been around for years
>>never show this extra valve?
>
>Because this is an unvented mains pressure hot water cylinder which
>requires the extra valve. Most hot water cylinders are gravity fed from
>a loft tank so this extra safety precaution is not required.

Sorry misread your original post, saw the vented but not the mains
pressure!

"My system is a totally standard open vented fully pumped Y-plan
system with an indirect **mains pressure** HW"


--

On Mon, 07 Nov 2005 04:22:50 -0800, Wingedcat wrote:

> I would like some help to understand my CH/HW configuration.
>
> My system is a totally standard open vented fully pumped Y-plan system
> with an indirect mains pressure HW cylinder and 5 radiators. However,
> there is one strange anomoly.
>
> As you might expect the mid position valve is on the flow pipe from the
> boiler, and its two outputs branch off to the Hot Water and the
> Radiator loop.
>
> However, there is a 2-way valve positioned between the HW port of the
> mid-position valve and the connection to the Flow port of the HW tank
> coil. This valve opens when the Call for Heat signal is given to the
> boiler, i.e. HW, CH or Both are demanded by the controller.
>
> At first glance this extra 2-way valve seems completely unnecessary.
> Why has it been put here? What harm can it do? I initially thought it
> could not do any harm if I leave it as it is. However, as the 2-way
> valve is connected to the Call for Heat signal to the boiler and not
> the live signal from the boiler to the pump, it could present a
> problen.
>
> For those that understand 3-way valves, if the valve gets left in the
> HW only position, and the boiler overheat stat fires up the pump,
> (which it does do for a few seconds every time the boiler has been on
> for a while then subsequently turned off), the pump will be asked to
> circulate the water in the HW coil, but won't be able to because the
> extra 2-way valve is shut, therefore straining the pump.
>
> Is this worth worrying about?
>
> Thanks for your help
>

The extra zone valve is correct. It is not permitted to use only a 3-port
valve to control the heating to an unvented cylinder. The cylinder has
been installed correctly.

If the boiler requires an a bypass for maintaining the flow after firing
then the primary circuit should be fitted with a permanent radiator
(e.g. towel warmer) or a smart bypass valve.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

On Mon, 07 Nov 2005 04:22:50 -0800, Wingedcat wrote:

> I would like some help to understand my CH/HW configuration.
>
> My system is a totally standard open vented fully pumped Y-plan system
> with an indirect mains pressure HW cylinder and 5 radiators. However,
> there is one strange anomoly.
>
> As you might expect the mid position valve is on the flow pipe from the
> boiler, and its two outputs branch off to the Hot Water and the
> Radiator loop.
>
> However, there is a 2-way valve positioned between the HW port of the
> mid-position valve and the connection to the Flow port of the HW tank
> coil. This valve opens when the Call for Heat signal is given to the
> boiler, i.e. HW, CH or Both are demanded by the controller.
>
> At first glance this extra 2-way valve seems completely unnecessary.
> Why has it been put here? What harm can it do? I initially thought it
> could not do any harm if I leave it as it is. However, as the 2-way
> valve is connected to the Call for Heat signal to the boiler and not
> the live signal from the boiler to the pump, it could present a
> problen.
>
> For those that understand 3-way valves, if the valve gets left in the
> HW only position, and the boiler overheat stat fires up the pump,
> (which it does do for a few seconds every time the boiler has been on
> for a while then subsequently turned off), the pump will be asked to
> circulate the water in the HW coil, but won't be able to because the
> extra 2-way valve is shut, therefore straining the pump.
>
> Is this worth worrying about?
>
Reading the other posts I would like to clarify something.
The only time this arrangement comes up is when an unvented cylinder is
retro fitted to an existing heating system that previously used Y-plan.
Anyone starting from scratch would use S-plan controls for sure. Unvented
cylinders are (IME) supplied with a zone valve anyway.

For this arrangement to work correctly the additional zone valve on the
indirect coil must be the sort with a a changeover micro switch (grey,
orange and white wire) so that HW satisfied signal is available for the
3-port valve.

The HW demand is taken through the tank stat to the 2-port valve which
then produces both HW demand and satisfied signals for the 3-port valve.

The boiler sounds like it really needs a bypass circuit which was lost
when the unvented cylinder was installed.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html